The flow of fluids through porous media, for example the production of fluids from wells, may be governed by three principle factors, namely the size of the flow path, the permeability of the flow path, and the driving force.
It is often necessary to stimulate the production of fluids from a subterranean formation when a well is not producing satisfactorily. The failure to produce is typically due to an inadequate, or a damaged, path for fluids to flow from the formation to the wellbore. This may be because the formation inherently has insufficient porosity and/or permeability or because the porosity and/or permeability have been decreased (damaged) near the wellbore during drilling and/or completion and/or production.
In sandstone reservoirs, production is often limited by either naturally occurring or induced damage that partially plug the pore spaces in the formation matrix near to the wellbore and hence reduce the effective permeability (flow capacity) of the formation. In such cases a stimulation treatment is performed by either injecting acid/chemicals to remove the damage in the formation, at pressures which do not exceed the fracture gradient of the formation (matrix stimulation), or bypassing the damage by propped fracturing treatment.
Fracturing is a treatment commonly used to stimulate production. In fracturing, a fluid is forced into the formation at a pressure above that at which the formation rock will part, to create an enlarged flow path. However, when the pressure is released, the fracture typically closes and the new flow path is not maintained unless the operator provides some mechanism by which the fracture is held open. There are two common ways of doing this. In conventional propped hydraulic fracturing, the fluid that is used to generate or propagate the fracture is viscous and carries a solid proppant that is trapped in the fracture when the pressure is released, preventing the fracture from closing. In acid fracturing, also known as fracture acidizing, the fracture is generated and subsequently treated with an acid.
The applications of fracturing treatments are limited by the ability to keep the fracture from propagating out of the reservoir and/or establishing communication with intervals containing or in contact with water. The success of matrix stimulation treatments is often limited by the compatibility of the treating acid/chemicals with the reservoir fluids, the volumes and types of specific chemicals required to fill the pore space, the limited contact between the treating chemicals and the damage in the pore spaces, as well as the associated time and cost.
Accordingly, there is a need for sandstone acid fracturing treatments to overcome the current limitations of both conventional matrix and propped fracturing treatments. By using a viscous dissolving fluid to create a permeable conduit that bypasses the near wellbore damage, without the need to use proppant.